The piezoelectrical effect is best described as the ability of some materials, such as piezo ceramics, to generate an electrical charge in response to a mechanical force, for example, being squeezed or pressed. The piezoelectric effect is reversible, in that materials exhibiting the effect also exhibit the reverse and/or inverse piezoelectric effect. Thus they change shape or size when excited by an electric charge. Although, the inverse piezoelectric effect has been well known and studied for some years, it is only relatively recently that commercial devices incorporating piezo technology have begun to find practical applications in everyday devices.
One major use of the piezo technology is in the development of piezoelectric motors. Examples of such piezoelectric motors are disclosed in U.S. Pat. Nos. 6,765,335, 7,973,451, 7,816,839, 7,737,605, and 7,211,919, the content of which is incorporated herein in their entirety. These types of motors are based on the frictional interaction of two parts, namely a rotor and stator, where one of these two parts is a piezoelectric element. As illustrated in U.S. Pat. No. 6,765,335, the piezoelectric element (or piezoelement) can be a rectangular plate with metal coatings on its main planar surfaces that has electrodes with leads connected to AC voltage excitation source. The piezoelement is pressed by a section of one of its sides against either a cylindrical rotor or a planar carriage surface. Specifically, a contact site on the piezoelement is pressed against a rotor or a carriage surface. The contact site is defined as the section of the surface area of the piezoelement that is in frictional engagement with the rotor or the carriage. The contact site executes elliptical motion to convert vibrational motion of the piezoelement into unidirectional motion of the rotor or of the carriage. The shape and arrangement of the electrodes are chosen to simultaneously excite higher-order modes of longitudinal vibration (the 2nd, 4th, etc.) along the length and the first-order mode of longitudinal vibration across the width of the piezoelement. The superposition of both vibrations gives rise to the elliptical motion of the contact site.
Although these piezoelectric motors have been promising, there are still certain drawbacks in utilizing elliptical motion of the piezoelement at the contact site. One such drawback is that one of the modes of vibration, i.e., the first order mode of longitudinal vibration across the width, produces a solely alternating motion of pressing one part of the motor against another, e.g. stator against rotor. As a result the energy is not transferred to the load, decreasing the efficiency of converting electric energy into mechanical energy. Consequently, the specific power and overall efficiency of the motor is unavoidably diminished.
In view of the foregoing, it would be desirable to provide a solution which overcomes the above-described inadequacies and shortcomings in the design of piezoelectric motors with improved efficiency and specific power.